The use of master cylinder assemblies for assisting in steering in the agricultural field is widely known. Actually, in agricultural vehicles, and in particular in tractors, a first and a second pedal are typically used in order to control braking of a left rear wheel and a right rear wheel, respectively, through a master cylinder assembly.
When the driver acts only on either the first or the second pedal, the master cylinder assembly controls braking of the left rear wheel or the right rear wheel only. By this feature, it is possible to assist in agricultural vehicle steering. Such a braking mode is also referred to as “steering braking” or “steering brake system”. If on the contrary the driver acts on both pedals, the master cylinder assembly controls braking of both rear wheels so as to carry out a “balanced braking” (i.e. with substantially the same intensity) of both wheels. For safety reasons, such a further braking mode must be ensured when the agricultural vehicle circulates on a road, also in case of failure of the braking system.
An example of such master cylinder assemblies according to the prior art is disclosed in Italian Patent No. 1064013. Such a document mentions the use of a balancing duct interconnecting the chambers of the master cylinders through the interposition of balancing valves, which are normally closed and are directly actuated by the displacement of the respective piston they are associated with. When the driver acts on one of the pedals, the piston connected to the pedal acted on is displaced and opens the corresponding balancing valve. Hence, if the driver acts on both pedals, the master cylinder chambers are in fluid communication with each other through the balancing duct. A pressure difference between the chambers, if any, is cancelled thanks to the connection established by opening the valves coming out in the balancing duct.
Yet, such a master cylinder assembly has some drawbacks.
A drawback is that the piston and the valve shutter are mechanically connected in series. Such an arrangement makes the master cylinder assembly bulky in longitudinal direction, with reference to the operating direction of the piston.
Another drawback is that the shutter seal is obtained by means of sealing gaskets that, in operation, slide against the surfaces of a bore or housing in the master cylinder in which the piston-shutter assembly is slidably mounted. Consequently, the seal performance of the shutter risks being degraded during use of the master cylinder assembly, since the gaskets risk being worn and damaged by the friction against such bore surfaces.
Moreover, the couple of sealing gaskets provided by the shutter have an additional drawback. When the balancing duct is closed at its ends by the balancing valves, the fluid contained therein can be exposed to a temperature increase during operation of the vehicle to which the master cylinder assembly is secured. In such situation, when the brakes are not used, a corresponding pressure increase occurs in the balancing duct and, especially at high operating temperatures, the fluid contained therein can exert considerable pressures onto the sealing gaskets. In this manner, the first time at least one brake pedal is operated, the risk exists of damaging the gaskets because of the high pressure of the fluid contained in the balancing duct. Such a situation results in compromising an effective and proper seal of the balancing valves.